ISPs wouldn't go with this simply because it'd require quite a lot of extra work on their end to make this happen for zero gain to them, and everyone knows ISPs do traffic-shaping for their own benefit, not for their customers' benefit. Also, end-users would just tag *everything* as to be prioritized, because they obviously don't want any of their traffic to be slowed down, so what would be the point? Besides, how the fuck would you even implement this for something that doesn't use a web-browser? Ask the ISP to list every possible network-protocol ever invented and all the ones still waiting to be invented, so you can click on them? That'd be one ginormous list to go through.
Also, I have to take offense at the whole "But treating all traffic the same isn't necessarily the best way to protect users." -- works fucking well over here in Finland, but then again, our ISPs aren't nearly as obsessed with overselling capacity. Maybe fix ISPs overselling their capacity, instead of trying to come up with workarounds that only harm end-users!